Kohta Ueno knows a lot about insulated roofs. As a researcher and engineer at Building Science Corporation in Westford, Massachusetts, Ueno has seen plenty of well-designed roofs, as well as plenty of rotten roof sheathing. For a building science researcher like Ueno, rotten sheathing isn’t a disaster; it’s data.
At a presentation on roofs at the March 2016 BuildingEnergy conference in Boston, Ueno shared the stage with Peter Marciano. Marciano is a roofing consultant; his half of the presentation focused on low-slope commercial roofs. Since GBA readers are more focused on residential work than commercial work, I won’t be reporting on Marciano’s presentation. (I can’t resist noting, however, that when Marciano develops specs for a roof replacement job, he almost always tries to include rigid foam above the existing roof sheathing. For a high-quality roofing job, Marciano said, “bring insulation into the equation.” He advised, “Lie to the owner. Tell him it’s required by code.”)
An unconditioned attic is easier than a cathedral ceiling
Most of Ueno’s presentation focused on unvented insulated roof assemblies. “Building Science Corporation has been doing a lot of work on unvented roofs in various configurations,” Ueno said. “We have a lot of field experience on these roofs.”
Ueno began his talk by praising unconditioned attics. “A ventilated attic is the classic approach. The attic is outside space with insulation down on the floor. The roof sheathing dries inward, to the ventilated attic. This is the most cost-effective way to build a roof. You end up with high R. It’s moisture-safe, even if there are roof leaks. The attic is safe from condensation in the winter, because the attic ventilation carries away any moisture. It’s a slam dunk.”
For this approach to work well, Ueno continued, “You need…
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